Electric Charge

Rub a balloon or styrofoam drinking cup against a wool sweater. It will then stick to a wall (at least on a dry day) or pick up small bits of paper. Why? The answer leads to the concept of electric charge.

Electromagnetic forces are one of the four fundamental forces in nature. The other three are gravitational, strong nuclear, and weak nuclear forces. The electromagnetic force unifies both electrical and magnetic forces. The magnetic forces occur whether the charges are moving or at rest. Electric charge is our way of measuring how much electric force an object can exert or feel.

Electric charge plays the same role in electric forces as mass plays in gravitational forces. The force between two electric charges is proportional to the product of the two charges divided by the distance between them squared, just as the force between two masses is proportional to the product of the two masses divided by the distance between them squared. These two force laws for electric and gravitational forces have exactly the same mathematical form.

There are however differences between the electric and gravitational forces. One difference is the electrical force is much stronger than the gravitational force. That is why the styrofoam cup mentioned above can stick to a wall. The electrical force pulling it to the wall is stronger than the gravitational force pulling it down.

The second major difference is that the gravitational force is always attractive. The electrical force can be either attractive or repulsive. There is only one type of mass, but there are two types of electric charge. Like charges will repel each other and unlike charges will attract. Most matter is made up of equal amounts of both types of charges, so electrical forces cancel out over long distances. The two types of charge are called positive and negative, the names given by Benjamin Franklin, the first American physicist. Contrary to what many people think, the terms positive and negative don't really describe properties of the charges. The names are completely arbitrary.

An important property of electric charges that was discovered by Benjamin Franklin is that charge is conserved. The total amount of both positive and negative charges must remain the same. Charge conservation is part of the reason the balloon and cup mentioned above stick to the wall. Rubbing causes electrons to be transferred from one object to another, so one has a positive charge and the other has exactly the same negative charge. No charges are created or destroyed; they are just transferred. The objects then have a net charge and electrical forces come into play.

Electric charge forms the basis of the electrical and magnetic forces that are so important in our modern electrical and electronic luxuries.